Tractors are known that have a hand operated demand lever that is used to set a minimum engine output power and a spring-biased foot pedal that can be used by the operator to increase the engine output power beyond the level set by means of the hand operated lever. Whichever of the hand lever and the foot pedal sets the higher power output level determines the governor line on the speed/load map of the engine along which the engine operates.
FIG. 1 of the accompanying drawings shows a typical speed/load map for diesel engines driving a tractor. The lines 110, 112 and 114 are three speed governor lines corresponding to three different desired engine speeds set by the hand lever and the foot pedal. When the engine is operating at part load, that is to say the operating or working point is below the maximum power curve 118, a slight decrease in load results in the working point moving from a first position (D) to a second position (E), simultaneously the engine speed rises from a desired value to a higher value while the governor reduces the amount of injected fuel to reduce output power and conversely a slight increase in load results in the working point moving from a first position (D) to a further position (F) and simultaneously the engine speed drops to a lower value while the governor increases the amount of injected fuel to increase the engine output power.
When the PTO shaft is connected to drive an implement, the conventional strategy used to control the transmission of the tractor is to maintain the engine operating near its maximum power, that is to say around point A on the maximum power curve which lies within a relatively flat region 120 of the curve 118. The total engine output power is shared between the drive wheels and the PTO. The PTO shaft always rotates at a fixed fraction of the engine speed but the loading on the PTO shaft may vary. The transmission control system acts to vary the power applied to the drive wheels, resulting in the vehicle aiming to move at the highest speed consistent with the engine operating at all times around working point A.
Because the maximum power curve is relatively flat around the point A, the engine speed can vary between the values B and C while still operating at nearly maximum efficiency. Consequently, the engine speed can vary quite significantly while still operating near is maximum output and efficiency.
PTO shafts are however intended to operate implements and ancillary equipment at preset speeds, typically one or other of 540 RPM and 1000 RPM. To achieve these speeds, the engine in the former case is required to run at a desired speed of, for instance, 1950 RPM and in the latter case at a desired speed of, for instance, 2154 RPM.
The load on the engine from the drive wheels is thus determined from the error in the engine speed and whenever the speed differs from a desired speed by a certain amount, referred to as the engine droop, a transmission ratio change is initiated.
The optimum engine droop depends on the implement that is being driven using the PTO shaft of the tractor. Some implements can tolerate considerable variation in the speed of the PTO while others are more sensitive and require tighter control over the speed of the PTO shaft. It is for this reason that some known tractors allow the operator to set the engine droop.